The present invention relates to an image generation apparatus for generating an image of a specimen, such as a wafer or the like, and more particularly to an image generation apparatus applicable to a scanning electron microscope for use in a pattern inspection apparatus for inspecting fine patterns on the order of submicron.
With the trend toward finer semiconductor devices, scanning electron microscopes have been often used for semiconductor inspection. In recent years, pattern sizes have become smaller and smaller. Such small-sized patterns entail a stricter demand for an inspection accuracy of a pattern inspection apparatus. In the pattern inspection apparatus, an electron-optics column emits a charged-particle beam that strikes a wafer, and secondary electron signals obtained are converted into an image. The image is compared with reference image data or CAD data for dimensions, shapes, etc. so as to evaluate whether patterns have been properly formed.
In order to inspect fine patterns on the order of nanometers, it is important to reduce a pixel size for an inspection image and to obtain a secondary electron image with less noise. It is thus necessary to suppress vibration and drift on the order of nanometers of an XY stage mechanism which supports a wafer.
Usually, the pattern inspection apparatus is installed in the same environment as a semiconductor manufacturing apparatus. Therefore, it is difficult to attenuate floor vibration and acoustic vibration with a passive anti-vibration table, an active anti-vibration table, and a sound-absorbing material, which are conventional techniques. Under such background, there has been proposed a method for directing a charged-particle beam to an accurate position by measuring vibration of a stage when an image is created and correcting an incident position of the charged-particle beam on the basis of an amount of fluctuation of the table.
As disclosed in Japanese laid-open patent publication No. 60-117721 and Japanese laid-open patent publication No. 2005-166538, there have been also proposed a method in which a vibration signal from an accelerometer, installed in an electron-optics column, is used to correct an incident position of a charged-particle beam. Japanese laid-open patent publication No. 2014-93153 discloses another approach which is to predict vibration of an electron-optics column from an image vibration and use the predicted vibration in correcting an incident position of a charged-particle beam. Japanese laid-open patent publication No. 2012-151053 discloses a frequency analyzing technique for an image vibration.
The above-described patent documents disclose a solution using a difference between two accelerometers mounted on a lower and intermediate portions, or upper and lower portions of an electron-optics column as a signal for correcting an incident position of a charged-particle beam, and a solution of determining frequencies of a low-pass filter and a band-pass filter on the basis of the natural frequencies of an electron-optics column. However, it is impossible to determine which of the natural frequencies of the electron-optics column is affected by a disturbance of the environment in which the pattern inspection apparatus is installed. Moreover, although the frequency of vibration caused by the environment of the site where the pattern inspection apparatus is installed does not vary greatly, the magnitude of the vibration vary at all times, which is problematic.